An Ecological Assessment of the Louisiana Tensas River Basin Chapter 4 
The small horizontal line drawn within the blue box 
indicates the median value. Any value outside of the 
10th and 90th percentiles is displayed on the graph as a 
light blue dot. This type of display was chosen rather 
than a simple mean ± standard deviation plot because a 
normality test indicated that many of the data distribu¬ 
tions for a given station or season were not normally 
distributed. In part due to the low sample numbers, 
which are indicated above the box and whisker illustra¬ 
tion, a mean value would be heavily influenced by outlier 
values. One extreme outlier nitrogen value of 5.9 mg/L 
was omitted from Figure 4.2 for the summer season from 
the Tendal station. This outlier, omitted for display pur¬ 
poses only, was included in the calculations in the sum¬ 
mary statistics given in the Appendix. 
Figure 4.2 shows seasonal differences exist for both 
nitrogen and phosphorus concentrations for all three 
stations with the exception of phosphorus concentra¬ 
tions from the Tendal station. In general, nutrient 
concentrations decline from the spring through the fall 
and then start increasing again in the winter. It is 
interesting that the Tendal station does not seem to 
fluctuate seasonally nearly as much as do the other two 
stations. Perhaps, this phenomenon is due to the 
proximity of agricultural fields to the water quality moni¬ 
toring station. Statistical summaries of the water quality 
data from the LDEQ stations can be found in the Appen¬ 
dix. 
To determine whether there were significant differences 
between the three stations, a Wilcoxon rank-sum (also 
known as Mann-Whitney U) test was performed. Again, 
a nonparametric statistic was used because the data 
are not normally distributed and the sample numbers 
are low. There were no significant differences between 
all three stations for total nitrogen. This was true for all 
seasons combined and for individual seasons. For total 
phosphorus there were statistically significant differ¬ 
ences between Tendal and Clayton and between 
Tendal and Winnsboro when all seasons were 
combined. When this analysis was performed on 
individual seasons, there were statistically significant 
differences between Tendal and Clayton for the summer 
and the fall and there were significant differences be¬ 
tween Tendal and Winnsboro for the fall. All other 
comparisons of station and season yielded nonsignifi¬ 
cant differences. A summary of this analysis is included 
in the Appendix. 
Figures 4.3 through 4.5 show the historical total nitrogen 
concentrations for all three locations on separate graphs. 
Figures 4.6 through 4.8 show the historical total phos¬ 
phorus concentrations. When there is a LDEQ station 
collocated with a USGS station, the points are plotted 
with different color symbols. It would appear that both 
nitrogen and phosphorus concentrations have increased 
slightly for the Clayton station when comparing data from 
the 1970s to the 1990s. It should be noted however that 
the data from the 1970s were exclusively collected by the 
USGS while the data from the 1990s were exclusively 
collected by LDEQ. It may not be appropriate to compare 
these data directly as there may have been differences in 
methodology. 
It is also interesting to compare the water quality from the 
three stream monitoring stations to the water quality in 
Lake Providence (Figures 4.9 and 4.10). Lake Provi¬ 
dence serves as the headwaters of the Tensas River and 
clearly the nitrogen and phosphorus levels are lower than 
they are at Tendal, Winnsboro, or Clayton. 
When evaluating the quality of the water in the Tensas 
River, one should look at how the data compare with any 
established criteria. To limit eutrophication potential to 
downstream waters, the EPA Quality Criteria for Water 
(1986) advises that total phosphorus levels should not 
exceed 0.1 mg/ L. This is also the criteria used by the 
EPA's Surf Your Watershed program. Clearly many of the 
values from the Tensas River samples exceed that level. 
We were unable to find a total nitrogen criteria for surface 
water. 
In summary the following can be said about nutrient 
levels in the Tensas River: They are higher in the stream 
water than they are in the headwaters, they are generally 
seasonal in nature, phosphorus levels are at a level 
where they could contribute to eutrophication, and there 
are some significant differences in phosphorus levels 
between Tendal and the other two monitoring stations. 
To perform a more thorough investigation of the water 
quality in the Tensas River Basin, a comprehensive water 
quality study that would characterize the water quality of 
all the subwatersheds within the basin should be de¬ 
signed and conducted. 
